Among other things, the study reveals several common denominators including a lower level of education, poorer integration into the labor market, higher levels of criminal activity, and stronger ties to the Maghreb and to sub-Saharan Africa than the average French citizen. Moreover, a qualitative analysis provides an understanding of the processes of radicalization and subsequent participation in terrorist activities. As such, it expounds the role played by group dynamics, the internet, and prisons. This study further illustrates the strains imposed on the judicial and penal systems by the jihadist phenomenon. Relapse is specifically explored, notably through the cases of individuals convicted of terrorism, who, after serving their sentence, launched attacks on French soil. The subject is all the more topical in light of the likely release from prison of some sixty individuals, sentenced for acts of terrorism, in the upcoming two years.